Star identification protractor



J. J. M CRACKEN STAR IDENTIFI CATION PROTRAGTOii Filed July 16 1920 2 Sheets-Sheet 1 Emily 3, i923. I EA387 J. J. M cRAcKEN STAR IDENTIFI CATION PRO TRAGTOR Filed July l6 1 920 2 Sheets-Sheet 2 itude the protractor may be used for all Patented July 3, 1923.

PATENT OFFICE-ii;

JOHN JAMES VMCGRA'OKEN, 01E NORFOLK, VIRGINIA.

STAR IDENTIFICATION PROTRACTOR.

Application filed July 16, 1920. Serial, No. 396,832.

To all whom it may concern;

Be it known that I, JOHN J. MCORACKEN, a citizen of the United States, residing at Norfolk, county of Norfolk, State of Virginia (whose post-office address is Magnolia and Carroll Streets, Larchmont, Norfolk, Virginia), have invented certain new and useful Improvements in Star Identification Pro-tractors; and I do hereby declare the following to be a full, clear, and exact description of the invention, such as Wlll enable others skilled in the art to "WhICh it appertains to make and use thesame.

This invention relates to a star identification protractor adapted for use in connec- .ion with the Hydrographic Ofiice star charts of the Navy Department and will definitely identify the navigational stars by means of their altitudes and azimuths. 1

Theoretically a protractor fo'reach degree of latitude is necessary and it is the salient object of this invention to provide a protractor which may be used for all latitudes between thirty and sixty degrees, on

the Hydrographic Oflice star chart of either the Northern or Southern Hemispheres, which will be sufficient for all general and practical purposes. a

It is for the purpose of obviating these repeated calculations for each degree of latitude, that the present protractor forming the subject matter of this invention has been devised.

With this improved protractor a navigator, by calculating the local sideraltime and with the local latitude as arguments, may

obtain the approximate altitude of any navigational star shown on the Hydro- .graphic Ofiice chart, as well as its aznnuth for any particular time desired, for example, sunrise and sunset. The advantage of this is to enable'the navigator to pick out stars whose azimuths are from fifty to ninety degrees apart and by setting the sextant for the altitude, he can generally pick up the star with said sextant while the hor1-' zon is good, even when the star cannot be seen with the naked eye. i a

of forty degrees and by constructing the altitude and azimuth curves for that latlatitudes between thirty and sixty degrees, the scale of the protractor being the same as the Hydrographic O'flicechart. Since, "the navigational stars are largeand sufficient-- 1y far apart, accuracy greater than five de-,

grees altitude and ten degrees azimuth is not necessary for identificat on.

The Hydrographic Ofiice chart beinga stereographic projectionon the plane of the equator, the altitude circles for-any zenith projections ata particular time will be projectedas ellipses, so that by projecting thc zenith of the observer for a latitude of forty degrees and bycalculating three pointsof the ellipse for each 'five degree change or latitude, we may draw in the projection of the altitude circles. By next calculating the declination and hour angle foreach five degrees of altitude and ten degree change of azimuth, indicating thesepointaand then 2 drawing a line through them on a 'fair curve the azimuth curves will be produced,

which is a method used inconstructing the various ellipses and curves on the protractor illustrated 111 the accompanying drawings,

to be hereinafter more fully referred to.

t The above'calculations have all been made with reference to the hydrographic star chart but instead of drawing these curves on the chart itself, they are put on transparent material such as celluloid, tracing cloth, or the like, which will allow the same to be repeatedly used by simply moving the curves to correspond to the changes in local sideral time and: local latitude, without each time going, through all of the above calculations; '1 I a i In order that the entire heavens, stereographically illustrated on the starachart, will be complete and accurate with respectv to thevarious planets, such as the sun, moon,

Jupiter, Mars, Neptune, and the like at the particular time of'taking the readings, I

have provided movable markers or semaphoreswhich may be moved about on the.

star chart and placed in their proper positions. It being a well known factthat the various other planets being variable, as well right ascensionof the sunand moonand the as their declinations. they are notshown 011m sion and declination the various markers may be put on the chart and thereby complete the entire heavens fornavigational purposes.

In the accompanying drawings Fig. 1 illustrates my improved protractor;

Fig. is a representation of a Hydrographic Oflice star chart of the Northern Hemisphere illustrating thereon, a few of the principal navigational stars with the protractor superimposed thereon and in preferred hydrographic star i'v'orkin'g relation "thereto and,

Fig. '3 illustrates an enlarged View of the i'espond to the projected altitude circles on the hydrographic star chart but are ellipses As hereinbefore stated the chart being a stereo-- graphic proj ectiononthe plane of the equatorythe various altitudes thereon are pro jected in circles, butthe altitude circles projected from a zenith not on the equator, will and not circles.

be projected as ellipses, as shown on the protractor illustrated inthe drawings, which protractor has been evolved by projecting the zenith ofthe observer on a latitude of forty degrees, which zenith is indicated at 2. By further calculating three points of the ellipse for each five degree change of latitude, as shown by the graduationsat 3, i and 5 we may draw in theprojections of the altitude circles, or in reality ellipses calculated from a latitude of forty degrees on the star chart.

In Fig. 3 I have illustrated a movable marker or semaphore v6 provided with a pointed projection or pin 7, which marker has indicated thereon one of the various planets, there being a marker or semaphore a for each planet. These markers are provided for use in connection with the Hydrographic Oflice star are for the purpose of indicating on the Hydrographic Office star chart the positions of said planets. These planets are not shown on the star chart for the obvious reason that erenceto the Nautical Almanac at the time desired, so that when the protractor is used form of one of the markers, each chart and the improved form of protractonherein described, and

the various planets can be indicated on the chart by sticking the markers in their proper places thereon, which will enable the navigator to complete the entire heavens and avoid confusion and prevent mistaking the planets for the larger stars, which is sometimes the case.

, In using this invention the Z of the protractor is placed on the hydrographic star chart, either on the Northern or Southern Hemispheres, over the intersection of the declination circle equalto the local latitude and the local sideral time equal to the right ascension, as shown on the chart, and by making the north and south lines of the protractor pass through the proper pole that is coincident with the north and south line of the star chart we may read oil. the approximate altitude and proper azimuth of any star shown on the chart.

By using the star chart in combination with a protractor having a movable zenith, the necessity of making any calculations between thirty and sixty degrees of latitude is not necessary for practical purposes. Due to the curves being constructed to forty degrees latitude, the error is greatest at places on the chart at right angles to the north and south lines, whereas near the meridian the error is aminimum.

Although a star chart of the Northern Hemisphere has been shown the protractor is clearly applicable to a star chart of the Southern Hemisphere, it being only necessary to make north the south when using,

hours, the zenith of the protractor being placed at the intersection of these two points and the north and south line of the protractor coincident with the north and south line of the star chart, the approximate altitude and azimuth of any star on the chart may be read, the same showing clearly through the transparent protractor.

Although the invent on has been illustrated and described in connection with the Hydrograp'hic Office star chart of the Navy limit the same to this specific form of chart, as the same may be used in'connection with other charts of a similar character.

What I claim is: c

1. A star identification protractor for use in connection with star charts, comprising a transparent plate provided with ellipses and intersecting curves, said curves radiating .Department, applicant does not wish to from the center of the protractor, the ellipses and curves representing the shape and relative positions of the imaginary altitude circles and azimuth curves of the earths sphere, stereographically projected on the plane of the equator from any zenith.

2. A star identification protractor for use in connection With star charts, comprising a transparent plate provided With ellipses and intersecting curves, the ellipses and curves representing the shape and relative positions of the imaginary altitude circles and azimuth curves of the earths sphere for each fivedegree change of latitude, stereographically projected on the plane of the equator from any zenith.

3. A star identification prot-ractor for use in connection with star charts, comprising a transparent plate provided With ellipses and intersecting curves, the ellipses and curves representing the shape and relative positions of the imaginary altitude circles and azimuth curves of the earths sphere for each five degree change of latitude, indicated by the ellipses, and of the declination and hour angle of each five degrees of latitude and ten degrees change of azimuth, indicated by the intersecting azimuth curves, said altitude ellipses and azimuth curves resulting from a stereographic'projection from any zenith on the plane of the equator.

A. A star identification protractor of trans parent material for use in connection with star charts, having drawn thereon ellipses and intersecting curves, the ellipses and curves representing the shape and relative posit-ions of the imaginary altitude circles and azimuth curves of the earths sphere for each five degree change of latitude, indicated by the ellipses, and of the declination and hour angle of each five degrees of latitude and ten degree change of azimuth, indicated by the intersecting azimuth curves, said altitude ellipses and azimuth curves being projected from a zenith on a latitude of forty degrees.

In testimon whereof I affix my signature.

' JO N JAMES MCCRACKEN. 

